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Arthur L. Rangno

Abstract

Images of frozen drops with pieces missing were collected on two days of airborne sampling in shallow supercooled stratiform frontal clouds in the coastal waters of Washington State. In those limited regions where ice appeared to be newly formed, ice fragments with rounded portions accounted for about 5% of the total ice particle concentrations. These results are in rough agreement with the body of literature on laboratory experiments concerning the freezing of drops in free fall that have suggested a modest, though not insignificant, role for the fragmentation of freezing drops on total ice particle concentrations when larger supercooled drops are present.

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Arthur L. Rangno

Abstract

A six-season, randomized-by-season cloud seeding experiment consisting of three seeded seasons and three non-seeded seasons was conducted by Colorado State University (CSU) during the middle and late 1960's in the Wolf Creek Pass region of the San Juan Mountains of southwest Colorado. The results of the seeding have been reported in a series of papers as having produced statistically significant increases in precipitation at Wolf Creek Summit when the 500 mb temperature was ≥−23°C. Furthermore, it has been reported that increases in precipitation produced statistically significant increases in the runoffs from three target watersheds when compared to the runoffs from three control watersheds.

In this paper the results of the Wolf Creek Pass Experiment (WCPE) are reexamined. It is shown that the three non-seeded seasons occurred during meteorological conditions which brought “warm aloft” (500 mb temperatures ≥ −23°C) storm days with unusually light precipitation over a wide region of Colorado, northern New Mexico, southern Utah and northern Arizona. This bias produced high values of seed/no-seed precipitation ratios at Wolf Creek Summit which led to the misperception of large increases in precipitation due to cloud seeding.

It is also shown that nearly all central and southwest Colorado watersheds with similar exposures to the target watersheds for the WCPE had high runoffs during the three seeded seasons compared to the three control watersheds chosen. Hence, the increases in runoff reported from the three target watersheds were part of a large-scale pattern due to natural causes rather than to cloud seeding.

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Arthur L. Rangno

Abstract

Some of the complexities of clouds and precipitation that have been encountered in field projects are reviewed. These complexities highlight areas of cloud microstructure and precipitation development that need to be better understood before adequate conceptual or numerical models of orographic cloud seeding can be developed. Some concerns about cloud sampling with regard to the evolutionary behavior of supercooled clouds from water to ice are also discussed.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

No abstract available.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

Extremely high ice particle concentrations developed rapidly in the ascending tops of maritime cumulus congestus clouds after drizzle drops had already formed below this level by the collision–coalescence mechanism. In one building cloud with a top temperature no colder than −8°C, the ice particle concentrations increased from 0 to >350 L−1 within 9 min. In another cloud with a top temperature no colder than −13°C, the ice particle concentrations increased from ≤1 to ∼1100 L−1 within 12 min. Subsequently, the ice particle concentrations in these clouds decreased, even though the cloud top temperature of one of the clouds continued to decrease to −23.5°C.

The mechanism responsible for these prodigious increases in ice particle concentrations is not clear. The concentrations built too fast to be explained by the riming-splintering mechanism as it is presently formulated. It is suggested that high ice particle concentrations might form in localized pockets of high supersaturation with respect to water.

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Arthur L. Rangno and Peter V. Robbs

Abstract

Two statistical experiments, carried out in Israel, appeared for a time to have provided a unique demonstration of the ability of cloud seeding to increase rainfall. In this paper the authors examine the possibility that both experiments were compromised by type I statistical errors (i.e., “lucky draws” or false positives). It is concluded that in the first Israeli experiment a type I statistical error produced the appearance of statistically significant effects of artificial seeding on rainfall 1) in the buffer zone and the center target area, 2) in the coastal region of Israel, a few kilometers downwind of the seeding, and 3) in portions of Lebanon, Syria, and Jordan.

Analysis of the second Israeli experiment using the original crossover design produced a null result. However, when the two target areas were evaluated separately, naturally heavier rainfall over a wide region on days when the north target area was seeded produced the appearance of increases in rainfall due to seeding in the north target area, and when the south target area was seeded, the appearance of decreases in rainfall due to seeding was produced.

Target-control (as contrasted with crossover) evaluations of the second Israeli experiment for the north target area alone foundered when control stations were selected from a relatively small region of anomalously low seed/no-seed ratios that was situated within a much larger region of high seed/no-seed ratios, which included Lebanon, Jordan, and most of Israel. Thus, the north target area seed/no-seed ratios are not an isolated, seeding-induced anomaly. On the contrary, it is the low seed/no-seed ratios of the northern coastal control stations, selected after the experiment began, that are anomalous in a regional context and are virtually the only stations that yield an apparently statistically significant effect due to seeding in the north target area.

It is concluded that neither of the Israeli experiments demonstrated statistically significant effects on rainfall due to seeding.

Considerations of the rainfall climatology of Israel, recent reports concerning the microstructure of clouds in Israel, aid the relatively small amount of seeding carried out in the first Israeli experiment support the view that seeding was unlikely to have had significant effects on rainfall. Contrary to previous reports, clouds in Israel contain large cloud droplets, precipitation-sized drops, and considerable concentrations of natural ice particles at quite high temperatures, all of which should obviate attempts to increase rainfall by artificial seeding in wintertime air masses.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Evidence is presented for the production of high concentrations of rather uniformly-sized ice crystals in a supercooled stratus cloud by a commercial, turbine, propeller-driven aircraft.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

No abstract available

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Peter V. Hobbs and Arthur L. Rangno

Abstract

The physical hypotheses for the cloud seeding experiments carried out by Colorado State University in the Colorado Rockies (Climax I and II and Wolf Creek Pass) in the 1960's are critically examined. Airborne measurements over the Rockies have shown that the concentrations of ice particles in the natural clouds are often much greater than originally assumed; consequently, the conditions under which it might be possible to increase precipitation by artificial seeding are probably much more limited than previously supposed. There appears to be no firm evidence to support the contention that 500 mb temperatures are a good measure of cloud-top temperatures over the Rockies. Finally, examination of a more extensive data set than previously used fails to substantiate the claim that precipitation over the Rockies decreases as 500 mb temperatures increase above certain values.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

No abstract available.

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